1. Field of the Invention
This invention relates to a projecting device used for observing a pattern, such as, for example, a film mask, printed board, or the like, and more particularly to a projecting device of a reflecting illumination type able to cope with an opaque object to be tested.
2. Description of the Prior Art
As a projecting device for the use of observation, there have heretofore been a transmission type in which an image formed by a light flux transmitted through an object to be tested, and a reflection type in which an image formed by a light flux reflected on an object to be tested is observed.
Although the transmission type is effective with respect to a transparent object to be tested such as film mask or the like, it cannot be used for an opaque printed board, etc.
Therefore, for the use of this type, there has been used a reflecting illumination type projecting device as conceptionally shown in FIG. 20.
The illustrated device comprises an illuminating optical system 10 including a light source 11 and a condenser lens 12, a projecting optical system 20 including a projecting lens 21, a screen 22 and relay mirrors 23, 24, a half mirror 30 disposed at angles with respect to an optical axis Ax.sub.2 of the projecting lens 21, and a stage 40 on which an object to be tested, such as a printed board, etc., is placed.
An optical axis Ax.sub.1 of the illuminating optical system 10 and the optical axis Ax.sub.2 of the projecting lens 21 are intersected with each other, and the half mirror 30 is situated on the intersecting point.
The screen 22 is symmetrical with respect to the optical axis Ax.sub.2.
A part of light flux emitted from the light source 11 is reflected by the half mirror 30 toward the object 41 and illuminates the object 41 through the projecting lens 21. The light flux reflected by the object 41 transmits through the projecting lens again and then transmits through the half mirror 30 to form an image of the object 41 on the screen 22.
In the case that the half mirror 30 is used as mentioned above, however, a quantity of light to form the image on the screen 22 is only 25% maximum of a quantity of light emitted from the light source 11 even on the assumption that the object 41 has a reflectance of 100%. In other words, loss in quantity of light is significant.
Furthermore, as the screen 22 of the conventional device is symmetrical with respect to the optical axis Ax.sub.2, it is susceptible to ghost when reflection is occurred on a lens surface of the projecting lens 21.
The ghost has two types; one is of the type that is condensed onto the screen and the other is of the type that is diverged, depending on curvature of a lens surface. The latter hardly affects the observation. Also, there is a great difference in degree of affection depending on degree of convergence. When ghost is occurred, contrast of an image of the object is degraded on the screen, and some parts of the image become difficult to observe.
In order to explain these ghosts, details of the projecting lens 21 will be described by way of two examples.
The projecting lens 21 shown in FIG. 21 is a telecentric lens toward the object, which is of six-piece structure. The projecting lens 21 has first to sixth lenses 21a.about.21f which are arranged in this order from the side of the half mirror 30.
According to this constitution of a lens, when a reflection is occurred on a surface at the mirror side of the fifth lens 21e, the ghost all returns to inside the screen as shown in FIG. 22. Moreover, in this case, as the ghost is hardly diverged in an optical path unto the screen 22, affection of the ghost appeared on the screen is significant. Particularly, this disturbs observation of the image of the object.
In the case that reflection is occurred on a surface at the object side of the sixth lens 21f, the ghost reaches the screen 22 in the manner as shown in 23 and disturbs observation. In this case, however, divergence of the ghost is greater than that of the case shown in FIG. 22, and as it is only a part of the ghost which reaches the screen, adverse affection to observation is less than that in the case of FIG. 22.
FIG. 24 shows another example of the prior art, in which a projecting lens 21' nontelecentric toward the object 41 is employed. This projecting lens 21' comprises first to fourth lenses 21g.about.21j. The relation of arrangement of the remaining component parts such as the half mirror 30, the screen 22, etc. is the same as that shown in FIG. 21.
In the projecting lens of FIG. 24, in the case that reflection in occurred on an object side surface of the third lens 21i, the ghost appears in a peripheral portion of the screen 22 as shown in FIG. 25. Similarly, in the case that reflection is occurred on an object side surface of the fourth lens 21j, the ghost appears in the central portion of the screen as shown in FIG. 26.
There have been several measures for reducing the ghost. One of the conventional measures employs an reflection reducing coating on a lens surface. In another measure, a lens is carefully designed at one stage of designing the lens so that ghost would not be conspicuous. However, none of these measures could fundamentally solve the problems.
In the above-mentioned two examples of the prior art, the same projecting lens to that of the present invention as will be described is employed in order to facilitate the comparison of the affection of ghost.